feat(core): add ai playground feature flag and remove model switch feature flag (#12934)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **New Features**
* Added support for the "AI Playground" experimental feature, including
new settings and localization entries.

* **Refactor**
* Renamed configuration and service references from "Model Switch" to
"Playground" across the AI chat and playground interfaces.
* Updated feature flag from "enable_ai_model_switch" to
"enable_ai_playground" for consistency.

* **Bug Fixes**
* The "Model" submenu in AI chat preferences is now always visible,
simplifying menu options.

* **Chores**
  * Removed outdated Claude model options from the chat prompt.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

packages/frontend/core/src/desktop/pages/workspace/detail-page/tabs/chat.tsx

@@ -47,7 +47,7 @@ export const EditorChatPanel = forwardRef(function EditorChatPanel(
     searchMenuConfig,
     networkSearchConfig,
     reasoningConfig,
-    modelSwitchConfig,
+    playgroundConfig,
   } = useAIChatConfig();
 
   useEffect(() => {
@@ -74,7 +74,7 @@ export const EditorChatPanel = forwardRef(function EditorChatPanel(
       chatPanelRef.current.searchMenuConfig = searchMenuConfig;
       chatPanelRef.current.networkSearchConfig = networkSearchConfig;
       chatPanelRef.current.reasoningConfig = reasoningConfig;
-      chatPanelRef.current.modelSwitchConfig = modelSwitchConfig;
+      chatPanelRef.current.playgroundConfig = playgroundConfig;
       chatPanelRef.current.extensions = editor.host.std
         .get(ViewExtensionManagerIdentifier)
         .get('preview-page');
@@ -109,7 +109,7 @@ export const EditorChatPanel = forwardRef(function EditorChatPanel(
     networkSearchConfig,
     searchMenuConfig,
     reasoningConfig,
-    modelSwitchConfig,
+    playgroundConfig,
   ]);
 
   return <div className={styles.root} ref={containerRef} />;